Raman light scattering has been successfully used in critical care situations to continuously monitor a patient's respiratory gases. This technique is based on the effect which occurs when monochromatic light interacts with vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is re-emitted as scattered light with lower energy and consequently lower frequency than the incident photon. In a similar manner, if the incident photon gains energy in the collision, it is re-emitted as scattered light with higher energy and higher frequency than the incident photon. Since these energy shifts are species-specific, analysis of the various frequency components present in the Raman scattering spectrum of a sample provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman spectral lines provides quantification of the gases present, providing suitable calibrations have been made. In this manner, Raman light scattering can be employed to determine the identity and quantity of various respiratory and anesthetic gases present in a patient's breath in operating room and intensive care situations.
In addition to critical care situations, Raman light scattering gas analysis can also be used in many industrial applications such as stack gas analysis for combustion control, process control, fermentation monitoring, and pipeline gas mixture control. This analysis technique can also be extended to meet environmental monitoring needs in many areas such as escaped anesthetic agents in the operating room, air pollution, auto emissions testing and submarine atmosphere monitoring.
Systems developed for analysis of gases in critical care situations utilizing Raman scattering typically employ gas cells which contain a sample of the patient's respiratory gas to be analyzed. The gas sampling cell is located either within the resonant cavity of a laser or outside the cavity. In an intracavity system, a laser beam is directed through the resonant cavity such that it intercepts the gas within the sampling cell. Raman scattered light from the gas analysis region within the cell is collected and analyzed by conventional discrete or spectrometric detectors to perform molecular gas analysis.
At the center of these systems is the Raman scattering phenomenon which produces scattered radiation of a very weak intensity. There has been a long felt need to improve the efficiency, i.e., the signal to noise ratio, and lower the cost of Raman scattering based instruments, but progress has been hindered by the weak intensity of the scattered radiation and the numerous sources of noise.
One source of extraneous background is the laser. This background is detrimental in that it adds noise to the signal. In a gas laser, the laser produces not only the desired laser light at a specific wavelength, but also a broad spectrum of light sometimes called plasma glow. Similarly, a diode laser emits light over a wide region of the wavelength spectrum, sometimes called spontaneous emission, in addition to the wavelength specific laser line(s). Some of the extraneous light from either type of laser, plasma glow or spontaneous emission, often overlaps the wavelengths of Raman scattered light of interest and can even overwhelm the Raman scattering.
Another source of system background is often due to the optical elements in the system. For example, if lenses or other optical elements are used to collect and shape light emitted by the laser, the interaction of the laser light with the lenses and elements may produce output fluorescence and Raman scattering characteristic of the materials in the lenses and elements. This output fluorescence and Raman scattering from the lenses and other optical elements can overwhelm out the Raman signals produced by the gas sample being analyzed. Optical filters, often used in Raman scattering systems, are also potential sources of extraneous fluorescence and Raman scattering. Typically, a filter comprises a filter coating which is deposited on a supporting substrate, for example glass. In order to prevent the filter coating from absorbing moisture from the air, it may be sandwiched between two glass plates and sealed around the edges. However, if such a filter is used in a Raman gas analysis system where the laser light interacts with the filter, the fluorescence and Raman scattering generated as the laser passes through the glass can produce extraneous light which can overwhelm the Raman scattering from the gas sample. This occurs even if a low fluorescing glass such as fused silica is used.
The device disclosed in European Patent Application publication number 0 557 655 A1 entitled "SYSTEM FOR COLLECTING WEAKLY SCATTERED OPTICAL SIGNALS" describes a system for enhancing the collection efficiency of a gas analysis system. This document discloses a system having a laser which illuminates an unknown gas contained by a long hollow chamber having a highly reflective coating. A laser beam propagates along the longitudinal dimension of the interior region of the long hollow chamber without contacting the reflective coating. The reflective coating, either on the inner or outer surface of a transparent hollow tube, is designed to reflect radiation which is scattered from the laser beam by the unknown gas contained within the long hollow chamber, thus enhancing the collection efficiency for light produced by the interaction of the laser beam with the gas sample, e.g. Raman scattered light. The laser beam is specifically prevented from interacting with the reflective coating since such interactions produce not only a reflected laser beam, but a weak collateral radiation, e.g. fluorescence and Raman scattering as discussed previously. This weak collateral radiation is unwanted because it tends to interfere with the measurement of the radiation scattered by the unknown gas. Thus, the approach taken by this reference for reducing the effect of the weak collateral radiation produced by the laser and by the interaction of the laser with reflective coatings and filters is to minimize such interactions by directing the laser beam along the longitudinal axis such that the laser beam does not interact with the reflective coating. Additionally, interactions with filters are eliminated by not having any filters in the laser beam path. While this approach may succeed in reducing collateral radiation due to interactions of the laser beam with the walls and/or reflective coating of the long hollow chamber, it also limits the kind of lasers which can be used. This is due to the tight collimation required to keep the incident laser light away from the walls and/or reflective coating of the long hollow chamber. It is difficult to produce a narrow, collimated beam with high powered diode lasers, thus reducing the intensity of light on the gas sample.
U.S. Pat. No. 3,556,659 entitled "LASER-EXCITED RAMAN SPECTROMETER" discloses a laser-excited Raman spectrometer in which a laser output beam having a very small diameter is projected along the length of a capillary sample cell, rather than being projected in a transverse direction. The beam is substantially coaxial with the cell and the resultant Raman scattered light travelling in the general direction of the cell axis is detected. This reference also emphasizes the importance of confining the laser beam to the capillary bore, with very little of it traversing the capillary wall. This is done to minimize the scattering of the laser radiation from the cell wall and excitation of fluorescence in the cell wall. It is also noted that aligning the laser beam with the capillary bore requires accurate alignment but is worth the effort because it minimizes fluorescence of the glass wall and scattering of light from the wall itself.
The device disclosed in European Patent Application publication number 0 557 658 A1 entitled "RAMAN SPECTROSCOPY OF RESPIRATORY GASES" describes a system for determining the composition and concentration of gases present in a patient's airway by measurement of the spectrum of Raman scattered light from these gases. The gases present are assumed to be drawn from a predetermined set of gases with known Raman scattering spectra, and the concentrations are determined by solution of a matrix equation Ac=b, where the c vector components are the unknown concentrations and the b vector components are determined from measurements of the Raman scattering intensities in a plurality of wavelength or wavenumber intervals. The gas sample flows through an optical waveguide similar to that disclosed in previously discussed European Patent Application publication number 0 557 655 A1. Collection optics including mirrors and filters direct the Raman scattered light from the sample into a diffraction grating spectrometer. The composition of the gas mixture is determined by analyzing the measured spectrum, which represents the sum of the spectra of the individual gases, weighted by the concentration of each gaseous component. Since this system utilizes components which are the same or similar to those disclosed in European Patent Application publication number 0 557 655 A1, i.e., the optical waveguide wherein the gas sample interacts with the laser, it also exhibits similar shortcomings. Additionally, as with any optical system, the collection and transportation of light from the optical waveguide to the spectrometer with mirrors and/or lenses is inherently inefficient.
What is needed is a gas monitoring system that: 1) reduces the level of non-laser light which is detected along with the desired Raman scattered light from the gas sample; 2) reduces extraneous fluorescence and Raman scattering produced by the interaction of the laser light with system components; and 3) efficiently couples the desired Raman scattered light from the gas sample to a detector or spectrometer.